Methods of and apparatus for dielectric heating



y 1955 c. JUHOLA 2,707,742

METHODS OF AND APPARATUS FOR DIELECTRIC HEATING Filed Nov. 24, 1951 3Sheets-Sheet l Rventor: Car/ Mafia/a I .55 Mi Alto; e:

y 3, 1955 c. JUHOLA 2,707,742

METHODS OF AND APPARATUS FOR DIELECTRIC HEATING Filed Nov. 24, 1951 I5Sheets-Sheet 2 May 3, 1

Filed Nov.

OSCILLATOR VOLTAGE OSCELLATUR VOLTAGE c. JUHOLA 2,707,?42

METHODS OF AND APPARATUS FOR DIELECTRIC HEATING 3 Sheets-Sheet 3 TIMEjfivem far:

Car! duke/a United States Patent Cifiice METHODS F AND APPARATUS FORDIELEQTRIC HEATING Carl Juhola, Manchester, Mass, assignor to UnitedShoe Machinery Corporation, Fiernington, N. 5., a corporation of NewJersey Application November 24, 1951, Serial No. 258,015

1i) Claims. (Cl. 2l9---10..77)

This invention relates to the treatment of materials by high-frequencyelectric energy, and more particularly to improved methods of andapparatus for controlling the application of the highfrequency electricenergy responsively to operating conditions during the heating cycle toenhance the rapidity of the heating operation and to protect theoscillator and work from damage by arcs.

"i he voltage at which an arc will form in a dielectric heating systemunder load is likely to vary widely from load to load even though theloads are apparently similar. The breakdown point may also vary duringthe course of heating of a given load. Accordingly, the usual procedurein dielectric heating has been to avoid arcs by limiting the voltage ofthe high-frequency energy to a value low enough so that no breakdown islikely to occur. This means tempering the voltage to a value which themost sensitive load will support without arcing. This value will bereferred to herein as a safe value. However, since the rate of heatingof a load varies approximately as the square of the voltage gradicnt,such operation represents a considerable loss in the potential rapiditywith which those loads which could withstand higher voltage could beheated.

Accordingly, an object of the invention is the provision of a method ofdielectric heating in which the voltage of the high-frequency energy isvaried in accordance with the breakdown conditions of the individualload so that the rate of heating of loads is not limited to thatprovided by the safe voltage.

In accordance with a feature of the invention, a method of dielectricheating is provided wherein the highfrequency energy is appliedthroughout the heating cycle in sub-cycles wherein the voltage isprogressively increased until are forms, the arc is quenched, forexample, by momentarily dropping the voltage, and the next sub-cyclecommenced by re-application of the highfrequency electric energy at avoltage below that at which the are formed. Where the load can support avoltage above the safe voltage, the average value of a voltage of aseries of such sub-cycles will be greater than the voltage forcontinuous operation at the safe value, and accor ingly the rapidity ofthe heating of such loads will be correspondingly increased.

Another object or". the invention is the provision of apparatus adaptedto carry out the method of the invention.

in accordance with a feature of the invention, dielectric heatingapparatus is provided for supplying high-frequency electric energy to aload throughout the heating cycle in sub-cycles wherein the voltage ofthe energy is progressively increased. By a feature of the inventionshould an arc occur during such progressive increase, the voltage isdropped to quench the arc, returned at a value below that at which theare formed, and another sub-cycle commenced.

The apparatus features improved means for sensing the formation of anarc, which means is here employed for controlling the supply of thehigh-frequency energy.

Cat

d it has previously been proposed to sense the formation of an arc bymeans responsive to the magnitude of a D. C. voltage derived from theoscillator feed-back or grid excitation, for example, from theoscillator grid current.

When an are forms, the additional loading imposed thereby reduces theexcitation and hence the grid current. 1 have found, however, that arcsmay occur under circumstances where the resulting grid current after thearc has formed is still Within the range covered by normal loadingrequirements. For example, a small arc may occur during a lightly loadedportion of a heating cycle which includes a portion normally moreheavily loaded than the loading imposed by the arc. However, i havenoticed that the drop in grid current caused by the formation of an arcis much sharper than changes in c= rent during a normal heating cycle,although the niagni" de of the grid current may remain within normallimits. Accordingly, a feature of the invention comprises the provisionof means responsive to a negative rate of change of the oscillator gridcurrent, or of a voltage derived from the oscillator high-frequencyexcitation, for sensing the formation of arcs. Tris means may beemployed to accompl sh various ends, for example, to drop and thenrecycle the oscillator anode voltage or to shut it oil entirely.

Since a negative rate of change o grid current is not present duringstarting of an osci ator, there is no need, such as there is inapparatus which cuts out below a pie-ietern'iined magnitude of gridcurrent, to provide time delay or time constant devices to permitstarting. Since any changes in the magnitude of grid current duringnormal operation are :dual, with consequent low rate of change, thecircuit may readily made insensitive to these.

Other features and advantages of the invention will best be understoodfrom cons ration of the following description taken in connection withthe accompanying drawings, in which Fig. l is a schematic diagram of anapparatus enibodying the invention;

Fig. 2 is a schematic diagrmn of apparatus embodying the invention inanother form.

Fig. 3 is a time-voltage graph of the sub-cycles of the heating cycleprovided by the apparatus of Fig. 1; and

Pig. 4 is a time-voltage graph of the sub-cycles of a heating cycleprovided by the apparatus of Fig. 2.

In Fig. 1 there is shown a dielectric heating system comprisingoscillator tubes 1i) Whose anodes 12 are grounded and Whose cathodes 14are connected to a tank circuit comprising an inductance 16 and heatingelectrodes 13 coupled to the inductance to by blocking condensers 2G.The cathodes of the tubes 15 may be indirectly heated as shown, or theymay be directly heated filaments. For simplicity the connections forheating the cathodes are not shown, since they form no part of thisinvention.

The grid circuit of the oscillator comprises the inductance 22 connectedbetween the grids 2d and having a center-tap connected to a center-tapon the inductance 16 through resistors 26, 27 and 23 which constitutethe grid resistance of the oscillator.

High voltage direct current for the anode circuit of the oscillator issupplied by a thyratron rectifier comprising tubes 39 having theiranodes 32 connected to the secondary of a transformer 34- Whose primaryis adapted to be supplied from a power line. The cathodes 36 areconnected for grounding the positive terminal of the rectifier, While acenter-tap of the secondary transformer 34, constituting the negativeterminal of the rectifier, is connected to the cathodes M of theoscillator via a resistance 28 and the inductance 16. Filtering for thishigh negative voltage is provided by a filter comprising an inductancell) and a condenser 42. The voltage drop across resistor 28 is employedto energize the anode circuit of an arc-sensing device hereinafterdescribed.

It will be understood that the voltage of the highfrequency alternatingelectric energy supplied to the electrodes 18 is determined largely bythe voltage of the direct current supplied to the oscillator by thethyratron rectifier. This direct current voltage is controlled byshifting the phase of alternating current supplied to the grids 44 withrespect to the voltage supplied to the anodes 32. For this purpose, analternating current derived from the power mains through a transformer50 is applied to the grids 44 through a conventional phase-shiftingcircuit comprising the resistors 52, 54 and condensers 56 and 58. Theresistor 52 has a shunt circuit comprising a variable resistor 6t? andcontacts 62 of a relay 64, while the resistor 54 has a shunt circuitcomprising a variable resistor 66, contacts 68 and a rheostat '70 havinga slider 72. The parameters of the phasing and shunting circuits are soestablished that the rheostat 70 toward its high-voltage position.Accordingly', the oscillator plate voltage rises progressively untileither the slider 72 reaches its limit or an are forms. In the lattercase, relay opens its contacts 90 deenergizing the motor 78 with itsclutch and the relay 4-, thus dropping the oscillator voltage andpermitting the slider 72 to return to its original position from whichit reascends for another sub-cycle of the heating cycle when thecontacts 94 are closed.

Fig. 3 shows the resulting form of the oscillator voltage. A typicalsub-cycle starts at a, an are forms b, and is quenched 0. Although inthe four sub-cycles shown the time during which the voltage is droppedfor rheostat 70 provides a means of controlling the voltage not go tozero. The rheostat range may advantageously I be from safe to maximum.

A voltage control-operating means is provided which comprises a motor 78connected in shunt with the relay 64. The motor is mechanicallyconnected to the slider 72 through an electromagnetic clutch 73 havingterminals 74 electrically connected to the motor terminals, and whenenergized, the motor drives the slider progressively up to its maximumvoltage position. When the motor '78 is not energized, the slider 72 isreturned progressively to its low voltage position by a spring St). Themotor, clutch and spring may conveniently be provided as a conventionaltiming device. The relay 64 and the motor 78 are arranged for connectionto the power mains through the contacts 913 of a relay 1152 and eitherholding contacts 92, or contacts 94. When the holding circuit is brokenby even momentary opening of the contacts 90, the relay 6 and motor 78remain unenergized until the closure of contacts 9 coincident with thereturn of slider 72 to the low voltage position.

In order to open the contacts 99 responsively to the formation of anarc, an electron discharge tube 1%, having the relay 102 connected inits anode circuit, has its grid circuit connected for biasing inaccordance with the rate of change of voltage across the resistor 27.For this purpose a condenser 108 and a resistor 110 are connected inseries across the resistor 27, the resistor 110 having its endsconnected respectively to the grid and the cathode of tube 1%. Thecondenser 103 and resistor 110 form what is commonly referred to as adifierentiating circuit. An abrupt change of the oscillator grid currentin the direction of a drop in the magnitude thereof produces a drop involtage across the ditferentiating circuit, and the resulting chargingcurrent in resistor 110 imposes a positive bias on the grid to increasethe conductivity of tube 1%. A variable resistor 112 is connected inshunt relation with the relay 1% and is adjusted so that the conductionin the tube 160 during normal operation of the oscillator isinsuflicient to cause the relay 192 to open its contacts.

In operation, after the tube cathodes have been brought up to operatingtemperature and the work disposed in operational relation to theelectrodes, a main power switch 125 may be closed to apply power fromthe line to the rectifier anode and grid circuits and to the relay 6 'Atthis time the motor 78 is energized, together with its magnetic clutch,and moves the slider 72 of voltage is caused progressively to increaseas before.

quenching the arc is exaggerated in comparison with the other times, itwill be readily seen that the average voltage is well above the safevoltage indicated by the dotted line.

Fig. 2 illustrates an embodiment which is similar to that of Fig. l,with the exception that recycling is performed after the lapse of apredetermined period after an arc has formed rather than upon the returnof the slider 72 to its original position, whereby the voltage at whicha sub-cycle commences after an arc may be a predetermined voltage belowthe voltage at which the arc formed. For this purpose, the relay 102 isprovided wtih a pair of holding contacts 130 by which the relay 102 isadapted to be energized from a condenser 132. When the relay 102 is notenergized, the condenser 132 is charged through contacts 134 and aprotective resistor 136. Provision is made for varying the length of theholding period by a variable resistor 13%. In this embodiment no holdingcontacts are provided for the relay 64.

in the operation of this embodiment, upon application of energy to themotor 78 and relay 64, the oscillator the occurrence of an arc, however,the relay 162 deenergizes the motor and drops the oscillator voltage byopening the contacts 99 for a period determined by the holding circuitof relay 102 as described above, after which the relay 102 again closesthe contacts and another sub-cycle is initiated at a voltagecorresponding to the position to which the slider 72 has returned duringthe interval. The amount by which the slider 72 returns at each cyclemay be adjusted.

Fig. 4 shows the form of the resulting voltage of the oscillator duringa cycle of heating. Again the voltage starts at an original voltage, a,which may be the safe voltage, and rises progressively until it isdropped at b upon the formation of an arc. However, the voltage is nowreturned to a voltage at below the previous breakdown potential butstill well above the original voltage a. In the next cycle, the voltagerises to a higher level b before arcing and accordingly the nextsub-cycle commences at a voltage higher than that of the precedingsub-cycle. Thus the average voltage will be somewhat higher thanprovided by the embodiment first described.

If desired in the embodiment of Fig. 2, the motor speed may be set at avery low figure or zero and the heating cycle started with the slider 72in its upper position. Then the voltage will automatically he steppeddown with each arc until no further arcs occur.

Where the Work can not tolerate any arcs, provision may be made, as byauxiliary electrodes, to confine the location of arcs to locationsharmless to the Work.

It is to be understood that my invention is not limited to theparticular embodiments disclosed but that changes or modifications maybe made Within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

l. The method of dielectrically heating loads which may vary as to thevoltage of the high-frequency electric energy which can be sustainedwithout harmful arcing, which method comprises supplying thehigh-frequency energy to the load throughout the heating cycle in aplurality of sub-cycles wherein the voltage is progressively increaseduntil an are just forms, the arc is quenched, and the voltage of thehigh-frequency electric energy is returned to an initial value for thenext subcycle, which value is less than that at which the are formedduring the previous sub-cycle.

2. The method of dielectrically heating loads, which method comprisessupplying the high-frequency electric energy to the load in a pluralityof sub-cycles of the heating cycle, each sub-cycle comprising aprogressive increase or" the voltage of the high-frequency energy, adropping of the voltage upon the formation of an arc to quench the arc,and a return of the voltage to an initial value for the next sub-cycle,said value being less than that at which the are formed during theprevious subcycle.

3. in a dielectric heating system including an oscillator, an electrontube having a grid circuit and an anode circuit, a differentiatingcircuit having input terminals and output terminals, means for derivingfrom the oscillator grid excitation voltage a direct current voltageproportional thereto and for supplying this direct current vol age tothe said input terminals, connections between the output terminals forsupplying the output voltage of the difierentiating circuit to said gridcircuit as a bias voltage, and means responsive to the current in thesaid anode circuit for controlling the output of the oscillator.

4. In a dielectric heating system including a self-biased oscillatorhaving a grid circuit through which the oscillator grid current flows,an electron tube having a grid circuit and an anode circuit, means forapplying to the grid circuit of said tube a voltage which variesaccording to the rate of change of the oscillator grid current, andmeans responsive to the flow of current in the anode circuit of saidelectron tube for controlling the energization of the oscillator.

5. In a dielectric heating system, a self-biased oscilla tor having agrid resistance through which the oscillator grid current flows, aresistance-capacitance differentiating circuit across a portion of thegrid resistance, an electron tube having the resistance of thedifferentiating circuit in its grid circuit, and a relay in the anodecircuit of the electron tube, said relay having contacts arranged tocontrol the energization of the oscillator.

6. In apparatus of the class described, an oscillator, a motor-drivencontrol for providing a progressive increase of the oscillator outputvoltage during a sub-cycle of the heating cycle, a relay responsive to areduction of the oscillator grid excitation, and means controlled bysaid relay for abruptly dropping the oscillator output voltage and forinterrupting the energization of said control for an interval and theninitiating another sub-cycle of progressively increasing output voltage.

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7. In apparatus of the class described in combination, an oscillator,means for controlling the magnitude of the output voltage of theoscillator, and an arc sensing device cooperative With said controllingmeans responsively to the formation of an arc of the oscillator outputto drop the magnitude of the output voltage to extinguish the arc andthereafter to raise the said magnitude to a higher value but below thatat which the are formed.

8. In apparatus of the class described, an oscillator, voltage controlmeans for determining the voltage of the oscillator output, a motorcooperating with the voltage controlling means for providing aprogressive increase of the oscillator output voltage, and a relayresponsive to a reduction in the oscillator grid current produced by theformation of an are for dropping the oscillator voltage to quench thearc and for initiating another sub-cycle of the heating cycle byre-applying the oscillator output at a voltage below that at which theare formed.

9. In apparatus of the class described, an oscillator, voltage controlmeans operable to vary the voltage of the oscillator output, voltagecontrol operating means energlzable to operate said control means toprovide a progressive increase of the oscillator voltage up to maximumvoltage, said operating means including means for operating said controlmeans progressively toward a minimum voltage position when not soenergized, means responsive to a change of condition of the oscillatorgrid current produced by the formation of an are for deenergizing theoperating means and for dropping the oscillator output voltage to quenchthe arc, and means for again energizing the operating means after a timeinterval.

10. In apparatus of the class described, a self-biased oscillator, avariable voltage supply for the anode circuit of said oscillator, meansfor operating said anode supply means to provide a progressivelyincreasing anode supply voltage for the oscillator, relay meansresponsive to a change of condition of the oscillator grid currentproduced by the formation of an are for dropping the volt age of theanode supply to a value insufficient to sustain the arc, and means forrecycling the operating means to initiate a further sub-cycle ofprogressively increasing anode voltage from a value less than that atwhich the are formed.

References Cited in the file of this patent UNITED STATES PATENTS2,391,085 Crandell Dec. 18, 1945 2,467,285 Young et al. Apr. 12, 19492,470,443 Mittelmann May 17, 1949 2,545,997 Hagopian Mar. 20, 19512,548,246 Walstrom Apr. 10, 1951

